User grabbing my Internal IP from WAN

I've setup a Wi-Fi LAN and my computer + another host are behind the router with NAT + SPI FireWall enabled. I was chatting and someone on my buddylist on Yahoo! Messenger (Buddies getting certain p2p rights with you) managed to grab my Internal LAN IP and not my router's WAN registered IP. I was just wondering how this could be? I don't see how this is possible considering that NAT should have retranslated all my computer's outbound packets stamping it's registered WAN IP on the packets instead of my Internal IP for all connections and requests. When inbound it would still go through the same translation only the other way around. I'm not understanding how my private LAN IP was grabbed and not the WAN? :confused:

Certain apps will run on the client itself so they grab the local IP and report it back to the "server". This is what probably happened to you.

What's funny is IIS, when configured just poorly enough, will do the same thing. A simple request will return internal addresses (or DMZ addresses) depending upon where someone parked the box.

Tiger is correct, and to add a few more pennies to the pot, there are a number of other ways this can happen. If you told us the P2P app and the version, I'm sure the exact cause will surface in this thread. ;)

If you were to go into a MSN (group) chat room it lists your IP upon entry. It lists the specific computer IP, by the way-----ie. 192.168.0.100 for mine, and not my router or ISP specific IP. It probably records the actual IP some where, and if it doesn't......oh well. (good luck finding the 192.168.0.100 addy since almost every router made by D-Link makes use of that internal IP)

If it really concerns you, you can force your router into giving you a different designated IP.